1. Field of the Invention
The present invention relates to an exposure apparatus and a device manufacturing method in which an image of a pattern is projection-exposed onto a substrate via a projection optical system and, in particular, to a liquid immersion type exposure apparatus.
2. Description of Related Art
Semiconductor devices and liquid crystal display devices are manufactured through the so-called photolithography technique, by which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus used in the photolithography process has a mask stage that supports a mask and a substrate stage that supports a substrate, and transfers the mask pattern, via a projection optical system, onto the substrate while successively moving the mask stage and the substrate stage. In recent years, there has been demand for higher resolution projection optical systems in order to handle the much higher levels of integration of device patterns. As the exposure wavelength to be used is shorter, the resolution of the projection optical system becomes higher. As the numerical aperture of the projection optical system is larger, the resolution of the projection optical system becomes higher. Consequently, the exposure wavelength used in exposure apparatuses has shortened year by year, and the numerical aperture of projection optical systems has also increased. Furthermore, the currently mainstream exposure wavelength is the 248 nm KrF excimer laser, but an even shorter wavelength 193 nm ArF excimer laser is also being commercialized. In addition, as well as resolution, the depth of focus (DOF) is also important when performing an exposure. The resolution R and the depth of focus δ are respectively expressed by the following formulas:R=k1·λ/NA,  (1)δ=±k2·λ/NA2,  (2)where λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. It can be seen from formulas (1) and (2) that if, to enhance the resolution R, the wavelength λ is made shorter and the numerical aperture is made larger, then the depth of focus δ becomes narrower.
When the depth of focus δ becomes too narrow, it becomes difficult to make the substrate surface coincide with the image plane of the projection optical system, and thus there occurs the possibility that the focus margin during the exposure operation will be insufficient. Accordingly, the liquid immersion method has been proposed, as disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. H10-303114, as a method to substantially shorten the exposure wavelength and increase the depth of focus. This liquid immersion method is designed to, by filling the space between the under surface of the projection optical system and the substrate surface with a liquid, e.g., water or organic solvent, form a liquid immersion region and thus by taking advantage of the fact that the wavelength of the exposure light in the liquid becomes 1/n times (n is the refractive index of the liquid and is generally about 1.2 to 1.6) of that in the air, improve the resolution and, at the same time, enlarge the depth of focus by approximately n times.
By the way, with the above-mentioned related art, there are problems as described below.
The immersion liquid type exposure apparatus disclosed in the above-mentioned Japanese Unexamined Patent Application, First Publication No. H10-303114, is configured such that a liquid bath is formed of a holder table, wall portions, etc. on a substrate stage, and a substrate is positioned in the liquid bath. In the case of such a configuration, when the substrate stage is moved, there arises not only the possibility that the liquid surface undulates and the liquid scatters, but also the possibility that the pattern image projected onto the substrate deteriorates due to the undulation of the liquid. In addition, pipings for supplying and recovering the liquid need to be connected to the substrate stage, which may adversely affect the movement accuracy of the stage.